This invention relates generally to bicycles and more specifically to drive mechanisms for bicycles. For decades, bicycles having multiple gear selection have conventionally employed a floating looped drive chain connected between a pedal gear and a rear wheel sprocket set. Gear shifting in these prior art bicycles has typically been accomplished through the use of a derailer that moves the drive chain from one rear wheel sprocket to another to effect a change in the drive ratio selected by the rider. The drive chain/derailer combination typically employed in these prior art bicycles suffers a number of disadvantages. Among them is a tendency of the drive chain and sprocket to snag and damage the rider's pant leg. Many riders choose to use a bicycle clip to keep the pant legs away from this mechanism. However, the chain itself attracts soil because it must be periodically lubricated to insure proper operation. As a result, this soil is often transmitted to the pant leg of the rider. Drive chains are susceptible to flipping off the various sprockets, and they tend to bind in positions off the sprockets. These conditions cause loss of drive power and require repair to reposition the chain. Finally, these prior art bicycle drive mechanisms are noisy as the result of the chain traversing the various sprockets during gear shifting operations initiated by the rider.
It is therefore the principal object of the present invention to provide a bicycle drive mechanism that totally eliminates the prior art drive chain/derailer mechanism and that instead employs a variable speed shaft drive mechanism that is smooth and efficient in operation and provides the user with an unlimited selection of drive ratios.
This and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a drive shaft that is coupled between the pedals and the rear wheel of the bicycle. A pinion gear at the forward end of the drive shaft is engaged by a ring gear mounted to the pedals to transmit drive power from the pedals to the drive shaft. A drive head at the rearward end of the drive shaft frictionally engages a circular flat drive pick-up plate mounted to the rear wheel of the bicycle to transmit drive power from the rotating drive shaft to the rear wheel. Infinitely variable speed control is implemented by a shifting fork actuated by the user to position the drive head at selected distances from the center of the drive pick-up plate. Alternatively, a second drive head/pick-up plate combination may be substituted for the ring gear/pinion gear combination driven by the pedals to provide even smoother operation. The second drive head may be positioned a fixed distance from the center of its associated pick-up plate or it may also be made variable, resulting in a dual variable speed shaft drive to provide a greater range of speed ratios selectable by the rider.